Apple Posts Mac Pro PCIe Flash Storage (SSD) Tech Note
Kudos to Apple for offering user-accessible flash storage (SSD), see Mac Pro (Late 2013): Removing and installing flash storage.
See updated discussion on 2013 Mac Pro: Choosing PCIe Flash Drive Capacity.
Upgrade solutions are likely to emerge by mid 2014 or thereabouts. Device quality, speed, longevity and capacity are all buying factors, and buyers are advised not to make assumptions on those points when upgrading.